If you’ve completed several marathons and feel ready for a bigger challenge, an ultramarathon might be just the thing for you. 
     While a 50K to 50-mile race might seem daunting, moving up from the marathon to an ultramarathon is actually easier than you might think — certainly, it’s easier than moving from a 10K to the marathon. 
     Here’s why: for races that take less than 2 hours to complete, your body can rely almost entirely on glycogen (blood sugar) for fuel.  Once you cross that 2-hour threshhold, your body will be forced to rely more and more on stored fat for fuel, or risk hitting the dreaded “Wall.”  Fat isn’t the body’s preferred source, since the process for metabolizing it is a bit more involved than it is for sugar, but fat is very calorie dense, and even a little goes a long way. 
     Long training runs and long races teach the body to access fat for fuel, and once your body learns makes this transition, it isn’t much harder for your body to simply keep going.
     Training for an ultra isn’t very complicated.  The basic idea is to use a marathon plan – like the ones available in Smart Marathon Training – to build base fitness and adapt the body to relying on stored fat as it’s primary source.  From there, you only need to do a bit more of the same, increasing the mileage of the long run. 
     The idea would be to continue on from marathon distance by doing a progressively longer run avery 2-3 weeks until that target is met — adding 3-4 miles to each long run — with a long endurance bike ride scheduled between these efforts. 
     On page 160 of Smart Marathon Training I include a chart indicating what the recommended longest run would be for target races, including ultramarathons.  For example, the longest training run for a 50-miler would be 35 miles, which should be completed 3-4 weeks before the target ultra.  
     If the idea of a 35-mile training run sounds awful to you, target a marathon as part of the run, and plan to get in a few miles before and/or after the race.  The marathon will supply you with support for your run, and you’ll also have a more interesting day running, complete with spectators cheering and hundreds or thousands of other people to talk to.
     But remember: this is a training run for you, not a race!  Do not try to run the marathon hard, or you will wind up needing downtime after the race to recover from your marathon, when really you should be just resting up for the ultra.
     When preparing for the ultra, you don’t need to focus very much on speed.  While it’s fine to maintain some of the speedwork that is included for the marathon plans, speed is less of a concern for the longer events, since the goal is more maintenance of a relatively easy pace for hour after hour than being able to produce efforts in the 80% range of your maximum heart rate.
     If the idea of an ultra still scares you a bit, remember that you were once worried about your first marathon, too.  Being a bit scared will keep you focused and honest about your training, you might end up surprised by what your body is truly capable of achieving.